Hello ΛΙΜΝΙΑΤΗ ΜΑΡΙΑ,
Thank you for the detailed reply. Since you have already created a Quizlet tab using the recommended steps, but it is still not showing up, it could be something related with backend side and to investigate this query further need more resources. As we belong to community, we have limited permission, so it is suggested to open ticket with Microsoft Technical team, so they can check remotely and assist you further. At this point, I highly appreciate your kind understanding and co-operation.
To open a ticket please see: Get support - Microsoft 365 admin | Microsoft Learn
Thank you for your valuable time.
Sincerely,Simbarashe | Community moderator.